All of the Queen's four children — Prince Charles, Princess Anne, Prince Edward and Prince Andrew — have arrived or are on their way to Balmoral, the royal family's estate in Aberdeenshire, Scotland.
Queen Elizabeth is under medical supervision there amid escalating concerns for her health, Buckingham Palace says.
"Following further evaluation this morning, the Queen's doctors are concerned for Her Majesty's health and have recommended she remain under medical supervision," the palace said in a statement released on Thursday night, Australian time.
"The Queen remains comfortable and at Balmoral."
The Duchess of Cornwall and Prince William are also at Balmoral.
The Duchess of Cambridge is reportedly remaining at home in Windsor to be with the couple's young children during their first week back at school.
Prince Harry, who is visiting the UK for work and charity commitments with his wife Meghan Markle, is also en route to Balmoral.
The Duke of Sussex was due to speak at the WellChild Awards in London on Thursday afternoon before flying home with Markle to be reunited with their children in California.
The Duchess of Sussex is not travelling to Balmoral and will reportedly remain in London on Thursday. She will not appear at the WellChild charity event as originally planned.
Barriers were reportedly being put up outside Balmoral Castle as members of the royal family gathered by the Queen's side and crowds congregated to the area.
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The latest announcement from the palace comes one day after the Queen postponed the Privy Council meeting after her doctors ordered her to rest.
The proceedings would have seen newly sworn-in UK Prime Minister Liz Truss take her oath as First Lord of the Treasury and new cabinet ministers sworn into their roles.
The Queen appointed Truss as prime minister at Balmoral on Tuesday, instead of travelling to London for the event.
It was the first time in the Queen's 70-year reign that the ceremonial transfer of power was held at Balmoral, rather than at Buckingham Palace.
She has been suffering from ongoing mobility issues and increasingly handed over duties to her heir, Prince Charles, and other senior members of the royal family in recent months.
The Queen has battled several health conditions over the past year, including COVID-19 and an overnight hospitalisation for what Buckingham Palace described at the time as "preliminary investigations.
Truss tweeted the whole country would be "deeply concerned" by the statement from the palace about the Queen's health, adding her thoughts were with her and the royal family.
Opposition leader Keir Starmer also said that his "thoughts are with Her Majesty The Queen and her family at this time."
"Along with the rest of the country, I am deeply worried by the news from Buckingham Palace this afternoon," he tweeted.
Meanwhile Commons Speaker Sir Lindsay Hoyle interrupted a speech during the energy debate to update MPs about the Queen's health.
"I know I speak on behalf of the entire House when I say that we send our best wishes to Her Majesty the Queen and that she and the royal family are in our thoughts and prayers at this moment," he said.
British Historian Andrew Roberts told MSNBC that the latest developments "only point in one direction".
"I'm afraid I believe the long life of Her Majesty the Queen is peacefully moving towards its close," he said.
"The way the royal family is converging on Balmoral; the fact she had Covid in February and that she's 96; the fact that she had to postpone at the last minute a Privy Council meeting that was on Zoom – all these things you take together and I'm afraid they only point in one direction."
Former Buckingham Palace press secretary Dickie Arbiter said there was every reason for people to be concerned about the Queen's health.
He said that formally appointing Ms Truss as prime minister in the week would have "taken a bit of a toll" on her majesty.
"The mobility issue is not something to be sniffed at and it would have taken all her strength to remain upright to have Boris Johnson and his resignation and then to meet with Liz Truss the incoming and new prime minister," Arbiter told the BBC.
Holding a walking stick, Her Majesty appeared frail but smiled as she greeted Britain's new prime minister for the traditional "kissing of the hands" ceremony on Tuesday.
Former BBC royal correspondent Richard Sumner said the careful wording of the palace statement suggests "something serious is possibly going on".
"The whole policy of the palace has always been understatement and I've always suspected that possibly there was more than a mobility problem concerned here," he told BBC News.
"When you're 96, of course you can't expect to have the health of a 25-year-old.
"Hopefully this is a passing storm that can be overcome but I think that judging from the careful wording of the palace something serious is possibly going on here."
Sumner added: "People who've seen her say that mentally she's sharp as a tack and very interested in everything still."
He said it seems "her body is giving out on her" – though it's not clear "quite in what way or how badly".
Queen Elizabeth II is England's longest-running monarch, having reigned for 70 years.